1. Field of the Invention
The present invention pertains to process control, and, more particularly, to signal quality in a process control system.
2. Description of the Related Art
One common process control technique is to sense some characteristic or parameter of the process and adjust an input signal to alter the process parameter. For instance, a process might be designed such that a particular process operation is performed at a specific temperature. When the process operation begins, the temperature control is set to the desired temperature. However, it is not uncommon for the actual temperature to vary from the desired temperature during the process operation and for such variation to have undesirable consequences. The actual temperature is therefore monitored so that, if it deviates from the desired temperature, the input signal can be modified to bring the actual temperature to the desired temperature. Note that this technique is used not only for temperature, but also for other types of parameters, such as pressure, flow rate, and volume.
In practice, the parameter is measured, or sensed, by a suitable type of sensor. The sensor typically outputs an electrical signal that is representative of the sensed parameter. A control system can then sample this signal at predetermined intervals and extrapolate the sensed parameter from the pertinent characteristic of the sample. For instance, in the temperature scenario introduced above, the temperature sensor might output an electrical signal whose voltage (or current) is proportional to the temperature is sensed. Every 0.05 seconds, a control system might sample that electrical signal. The control system then determines from the voltage of that signal in that sample what the actual temperature was at the time it was sensed.
This control technique relies on the accuracy of the information conveying the actual, or measured, parameter. One problem in this respect is electrical “noise” in the signal the control system samples. The electrical noise alters the signal characteristics. If the noise is severe enough, the control system cannot accurately determine what the sensed parameter is, thereby leading to degraded process control. Electrical noise arises from many sources and is largely unavoidable. However, it can be mitigated.
There are many ways to reduce or mitigate electrical noise. One technique filters out unwanted frequencies in the sampled signal on the presumption that they are noise. More particularly, a low pass filter with a fixed cut-off frequency can be used to eliminate noise when the noise frequency spectrum is beyond the specified cut-off frequency. However, the noise signal could have a wide and dynamic frequency spectrum and sometimes it is very time consuming to figure out the noise spectrum range in a certain process. The noise spectrum could also be different in the same process if the data acquisition system is located in a different place. If the filter has a very low cut-off frequency, it could eliminate the majority part of the noise, however, it could also slow down the sensor response time in the data acquisition system significantly.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.